Harmonics Flashcards
What does harmonics equal?
Two times the operating frequency
What is another name for harmonics and what is harmonics a result of?
Beam dynamics
Results from propagation through tissue
What is wave distortion?
As the ultrasound beam propagates several harmonic frequencies are produced as a result.
As density increases, velocity will do what?
Decrease
As compressibility increases velocity will do what?
Increase
What is wave distortion dependent on? (3)
- Intensity of the beam
- Distance travelled
- Nature of the tissue
How are harmonic frequencies produced?
Non-linear (a little different when they come back each time)
How are fundamental waves produced?
Linear
How does high pressure affect compression and velocity?
Higher pressure causes higher compression and therefore increased velocity
How does low pressure affect compression and velocity?
Lower pressure causes the tissue to expand (less compressed) and therefore the velocity is lower.
Harmonics can benefit the image in what three ways?
- Narrower beam (harmonics are best produced from most intense part of beam)
- Eliminated grating lobes (grating lobes are too weak to produce harmonics)
- Reduced/eliminated reverberation
What was developed to eliminate the fundamental frequency and allow the harmonic signal to pass through the beam former
Bandpass filtration
When we say that the fundamental freqeuncy was eliminated with bandpass filtration, what can we actually think of?
Having a filter, the fundamental frequencies are not allowed through the beam former, only the harmonic ones
For bandpass filtering to work, the bandwidths must do what?
The fundamental (Fo) and second harmonic (2Fo) bandwidths must fit within the overall transducer bandwidth without overlapping (more narrow bandwidths)
Narrow bandwidth means what in terms of pulse and axial resolution?
Longer pulse and poor axial resolution
What is a technique used to filter out the fundamental frequency and leave only the harmonic signal while maintaining a wide bandwidth?
Pulse inversion
What is pulse inversion?
A pulse is sent out followed by a second inverse pulse allowing filtration of the fundamental frequency while maintaining a wide bandwidth.
How does pulse inversion work?
Fundamental frequencies are cancelled out when they are added together due to being linear, while harmonic frequencies prevail due to being non-linear (distorted).
What are contrast agents?
Liquid suspensions that can be injected into the blood stream to improve the return of echoes for enhanced visualization of blood/tissue.
What are three musts for a contrast agent to be successful?
- Good back scatter
- Have to be small enough to pass through capillaries but large enough to produce echoes
- Have to be strong/stable enough to pass through the heart for several cycles to allow adequate imaging time
What are the majority of contrast agents made of?
Microbubbles of gas contained within a protein, lipid, or polymer shell
What is coded excitation?
A machine program that allows the visualization of blood flow in 2D without doppler (avoids associated artifacts) by digitizing ultrasound pulses to help reduce noise and improve overall image quality.
What kind of frame rate does coded excitation give you?
Higher frame rate
What enhancements does coded pulse allow? (7)
- Higher frame rate
- Increases sensitivity (ability to pick up weak echoes)
- Suppresses noise
- Improved spatial resolution/small lesion identification
- Improved penetration
- Improved contrast/subtle contrast difference detection
- Helps with TDS patients (improved productivity, better IQ and probe versatility)
What does coded excitation do for penetration and contrast?
Improves it (improves subtle contrast differences)
What is the purpose of the gas and the shell in microbubble contrast?
The gas creates a large impedance mismatch (scatter)and the shell keeps the gas from dissolving into the blood
Why are contrast agents useful?
They can improve lesion detection and increase doppler signals
What happens to microbubble contrast when scanning with 100% power?
The bubbles collapse and the gas is dissolved
How is microbubble affected by 0% output power?
It is static
What is the ideal output power when scanning microbubble contrast and why?
75%
The bubbles will produce very intense harmonic echoes which increase the contrast between the contrast and the tissue
How does increasing power affect microbubble contrast?
0% = Static
0 - 50 = Resonance
50 - 100 = Harmonic production
100% = Implosion
What mode does coded excitation use?
B-mode
How does coded excitation work?
- Digital encoder codes the pulses so the weak blood echoes can be differentiated from noise
- Decoder enhances the flow signal
- Flow and tissue displayed in B-mode
How does coded excitation differ from traditional colour doppler?
- CE gives simultaneous tissue and flow info unlike the transposed overlay of doppler onto 2D image
- Intuitive B-mode display gives full field of view
- No separate firings for flow allows a higher frame rate
